The preservation of blood and cellular biological thermolabile substances involves storage at very low temperatures. Thermolabile substances are substances that are easily altered or decomposed by heat. They can be contained in bags of plastics material; however, storage at very low temperatures creates stresses in the plastics materials and their joints and accordingly the bags used for this purpose must meet stringent requirements.
U.S. Pat. No. 5,928,214 described a bag of plastics material for containing biological liquid samples in particular for the cryopreservation of such samples, the bag being of the type made of facing plastics films joined around a sealed peripheral edge. The facing plastics films define a volume for containing a sample, in particular with several compartments communicating with one another by heat-sealable regions. This bag was described in connection with a system for concentrating white blood cells wherein the bag containing stem cells was divided into compartments limited by a heat seal that divided the stem cell freezing bag into two intimately attached but independent white cell containers with heat seals at the dividing locations. The larger main chamber keeps the bulk of the white cells and a smaller chamber is used for storage of a smaller fraction which can be separated from the main compartment without thawing.
U.S. Pat. Nos. 6,146,124; 6,232,115 and 6,808,675 describe respectively a mold, a bag useful for the cryopreservation of thermolabile substances and a method to manufacture the bag. The method uses a first mold having a portal-shaped recess and a recess with a planar surface having a radiused periphery circumscribing the planar surface and a peripheral ledge that circumscribes the radiused periphery. A sheet of plastics material is placed over the first mold and caused to conform to its shape. This formed sheet—which forms a half-shell of the bag—is then placed facing a similar formed sheet, or a flat sheet, and the two sheets are joined together by high frequency sealing around the periphery.
This manufacturing method is thus done in three different steps: individual pre-shaping of the two films with two different molds, one per film; positioning the two shaped films and the connector/tubes; and sealing of the bag borders with the connector and tubes
This process allows manufacture of a bag with a 3-D shape therefore a reduced space for the storage. However, the process requires the aforementioned three steps, and if different bags with different compartments are to be produced, different two-part molds are needed.
Bags can also be formed by folding over a planar film and joining the peripheries by a seam.
The standard process for making flat plastic bags is by high-frequency welding around the periphery of two flat films. The tool is composed of two matrices which seal the borders of the film and the connector and tube sealing is included in the same step. This procedure is simple, however the bag keeps a 2-D shape and its storage capacity is limited. This therefore does not meet the specifications of certain types of bags which require a volume that fits in a specific protection cassette, for instance with 25 mL capacity for cryogenic storage. This simple process is thus inapplicable for bags which must have a given volume.